1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and to a method of lithographic printing using this lithographic printing plate precursor. More particularly, the present invention relates to a lithographic printing plate precursor that enables so-called direct platemaking by laser scanning based on a digital signal from, for example, a computer; to a method of lithographic printing in which printing is carried out by direct development of the aforementioned lithographic printing plate precursor on a press without going through a development process step; and to a method of lithographic printing in which printing is carried out after the aforementioned lithographic printing plate precursor has been subjected to a development process step using a gum solution.
2. Description of the Related Art
A lithographic printing plate typically comprises an oleophilic image area that is ink receptive during the printing process and a hydrophilic nonimage area that is fountain solution receptive during the printing process. Lithographic printing is a method that utilizes the fact that water and oleophilic ink repel each other: differences in the ink attachment behavior are produced on the surface of the lithographic printing plate by using the oleophilic image areas on the lithographic printing plate as ink receptive areas and using the hydrophilic nonimage areas on the lithographic printing plate as fountain solution receptive areas (areas not receptive to ink). After ink uptake has been brought about only in the image areas, the ink is transferred to the receiving medium, e.g., paper.
A lithographic printing plate precursor (PS plate) comprising an oleophilic photosensitive resin layer (image recording layer) disposed on a hydrophilic support has heretofore been widely used to produce the aforementioned lithographic printing plate. Platemaking is typically carried out by a method in which the lithographic printing plate precursor is exposed to light through an original image, for example, a lith film, after which the areas forming the image areas of the image recording layer remain while nonimage areas on image recording layer is dissolved and removed by an alkaline developing solution or an organic solvent to expose the hydrophilic surface of support, thus yielding the lithographic printing plate.
The conventional platemaking process for lithographic printing plate precursors has required a step in which, after photoexposure, the unwanted image recording layer is dissolved and removed by, for example, a developing solution adapted to the image recording layer; however, a concern with these separately conducted wet processes has been to render them unnecessary or to simplify them. In particular, attention to the global environment has in recent years caused the disposal of the waste solutions discharged in association with these wet processes to become a major issue for the industrial sector as a whole, and as a consequence there has been an even stronger desire to address the aforementioned concern.
In this context, the method known as on-press development has been introduced as a convenient platemaking method. In on-press development, an image recording layer is used that enables the removal of unwanted areas of the image recording layer to be carried out during an ordinary printing process: after photoexposure, the lithographic printing plate is obtained by removal of the unwanted areas of the image recording layer on the press.
The following are examples of specific methods of on-press development: use of a lithographic printing plate precursor that has an image recording layer that can be dissolved or dispersed in the fountain solution, in the ink solvent, or in an emulsion of the fountain solution and ink; mechanical removal of the image recording layer by contact with rollers or the blanket cylinder on the press; mechanical removal of the image recording layer by carrying out contact with rollers or the blanket cylinder after the cohesive strength within the image recording layer or the adhesive force between the image recording layer and support has been weakened by penetration by, for example, the fountain solution or the ink solvent.
Unless stated otherwise, in the present invention, the “development processing step” refers to a step in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer that have not been exposed to the laser, wherein this removal is effected by contact with a fluid (typically an alkaline developing solution) using an apparatus (typically an automatic developing apparatus) outside of the press, and “on-press development” denotes a step and a method in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer that have not been exposed to the laser, wherein this removal is effected by contact with a fluid (typically the printing ink and/or fountain solution) using the press.
In another convenient platemaking method, removal of the unwanted areas of the image recording layer is carried out with a gum solution (this is typically an aqueous solution that contains a hydrophilic resin), i.e., gum development is carried out, and subsequent to this contact with printing ink and fountain solution on the press is executed in a conventional manner and printing is carried out.
At the same time, digital technology, in which the image data is electronically processed, stored, and output using a computer, has become widespread during the last few years, and various new image output methods have entered into practice in association with this digital technology. Accompanying this, interest has been growing in computer-to-plate (CTP) technology, in which the digitized image data is carried by a highly convergent beam of radiation, for example, laser light, and the lithographic printing plate precursor is subjected to a scanning photoexposure with this light in order to directly produce the lithographic printing plate without going through lith film. As a consequence, the appearance of lithographic printing plate precursors adapted to this technology has become a technical problem of the utmost importance.
Thus, as described in the preceding, based on global environmental concerns and the need to adapt and conform to digital technology, there has recently been an even stronger desire than before for a simplification of platemaking technology, for its conversion to a dry technology, and for its conversion into a processless technology.
A light source and image recording layer that support handling in a light room or under yellow illumination are required for the simplification of the platemaking process as described above, or for its conversion to a dry procedure, or to make it processless. This requirement arises from the fact that the image recording layer exhibits photosensitivity after exposure because it has not been fixed by a development process, which creates the potential for fogging in the interval up to the printing step. Solid-state lasers (e.g., the YAG laser and so forth) and semiconductor lasers that emit infrared radiation at wavelengths of 760 to 1200 nm are very useful as such laser light sources because they can be inexpensively acquired as small, high-output devices. UV lasers can also be used.
An example of an on-press-developable lithographic printing plate precursor that is image-recordable with such an infrared laser is described in Japanese Patent No. 2,938,397, which describes a lithographic printing plate precursor in which an image-forming layer having particles of a hydrophobic thermoplastic polymer dispersed in a hydrophilic binder is provided on a hydrophilic support. In its essential features, Japanese Patent No. 2,938,397 describes the possibility of exposure of this lithographic printing plate precursor with an infrared laser to bring about image formation by the thermal coalescence of the hydrophobic thermoplastic polymer particles; mounting on the press cylinder; and on-press development by the fountain solution and/or ink.
This method of image formation by coalescence by the simple thermal fusion of microparticles does have excellent on-press developing characteristics, but is compromised by a very weak image strength and an unsatisfactory printing durability.
A lithographic printing plate precursor comprising a hydrophilic support bearing microcapsules that hold a polymerizable compound is described in Japanese Patent Application Publication Nos. 2001-277740 and 2001-277742.
In the lithographic printing plate precursor described in Japanese Patent Application Publication No. 2002-287334, a photosensitive layer containing an infrared absorber, a radical polymerization initiator, and a polymerizable compound is disposed on a support.
Due to the high density of chemical bonds in the image regions, these polymerization reaction-based methods are characterized by a relatively good image strength in comparison to image areas formed by the thermal fusion of polymer microparticles. However, an intermediate layer has had to be disposed between the support and the image-recording layer in order to provide a satisfactory printing durability, capacity for on-press development, and scumming behavior.
The use of a water-soluble resin that confers hydrophilicity for an intermediate layer tasked in such a manner is generally known. The on-press-developable lithographic printing plate precursor described in Japanese Patent Application Publication No. 2005-125749 is provided with an intermediate layer of a polymer that has a hydrophilic group, a polymerizable group, and a group capable of adsorbing to the substrate.
However, the scumming behavior in the nonimage areas is still unsatisfactory even when an on-press-developable lithographic printing plate precursor is provided with these intermediate layers, and there has been a requirement for additional improvements with regard to scumming that takes the form of circular spots of from several μm to several thousand μm (referred to hereafter as spot scumming).
In addition, in a step that precedes mounting of the printing plate on the press in the case of lithographic printing plate precursors, the image on the printing plate is inspected and identified in order to determine whether image recording has been performed in accordance with the purpose of the printing plate. With the usual lithographic printing plate precursors that are accompanied by a development process step, the image recording layer is typically provided with color in advance. This facilitates identification of the image after platemaking (after the development process) and before printing (before mounting the printing plate on the press).
However, with on-press-developable or processless (developmentless) lithographic printing plate precursors, which are not accompanied by a development process prior to printing, there is no image on the printing plate at the stage where the printing plate is mounted on the press and inspection of the plate therefore cannot be performed. In particular, the ability to distinguish whether a register mark, which functions as a guide for registration in multicolor printing, has been written out is crucial for the printing process. Due to this, there is a requirement with on-press-developable or processless (developmentless) lithographic printing plate precursors for a means of checking the image at the stage where exposure or heating has occurred, that is, the exposed region or heated region must develop color or must decolorize.
Printing plates have been introduced that employ, as a print-out agent, a compound that generates acid, base, or a radical under the action of light or heat and a compound that changes color when it interacts with the generated acid, base, or radical (refer, for example, to Japanese Patent Application Publication No. Hei 11-277927). The utilization of the color change of a thermodegradable compound as a print-out agent has also been proposed for a direct-writing lithographic printing plate precursor that has a heat-sensitive layer (refer, for example, to Japanese Patent Application Publication No. 2000-335129). The use as the print-out agent of a thermodegradable dye that has a thermodegradation temperature no greater than 250° C. has also been proposed (refer, for example, to Japanese Patent Application Publication No. 2003-191657).
According to these cited technologies, colorization or decolorization is produced in the exposed areas, and, while the ability to check the image is improved to some degree, these technologies have been associated with various problems. For example, color development sufficient to check the image may not be obtained; dye that has undergo color development, or dye that has not been degraded by exposure and hence has not undergone decolorization, can stain the fountain solution and influence the finish of the printed material; insoluble dye may remain present in the ink or fountain solution; and the dye can react with components in the ink or fountain solution to produce a precipitate, i.e., scum.